The performance of ground based moving target indicator (MTI) radars depends heavily on the stability of the transmitter. A spectrally pure STALO signal will be degraded by its passage through a high power RF amplifier. If the transmitted signal is not stable enough, the radar's ability to reject ground clutter and returns from stationary objects will be degraded. The uniformity of each pulse allows the radar to reject such clutter and stationary returns. The object of this invention is to test whether the transmitted signals of a unit under test meet the rigid specifications of modern MTI radars.
Modern radars rely heavily on multiple coherent pulses for reduction of clutter. This has become more important in the case of military radars due to the development of "stealth" targets with small radar cross sections which must be detected and tracked through heavy clutter, which can be much larger that the signal.
The use of Moving Target Indicator (MTI) circuitry to cancel clutter continues from the early days of radar to this day, although today it is more flexible and accurate. In a conventional MTI radar, pulses received from the target are delayed in time until the arrival of a second pulse, and then the first delayed pulse is subtracted from the second pulse. The clutter, which is stationary, is thus canceled and the target appears. The amount of cancellation of clutter is called the cancellation ratio CR, defined as: CR=(Clutter in)/(Clutter out). When signal integration schemes are used to gain additional detection, this becomes the Improvement Factor (IMF): IMF=(Signal Out)/(Signal IN) =CR Subclutter visibility is another term used. It is the size target detectable below clutter, expressed as a ratio (in db). While suitable for comparing radar systems, it is dependent on target type and fluctuation, clutter type, false alarm rate, probability of detection, and threshold levels. Therefore, 2 pulse cancellation ratios are a better transmitter stability benchmark.
Clutter Ration (CR) is limited by the degree that transmitted pulses do not perfectly replicate. When a transmitter power supply changes from pulse to pulse, this produces phase and amplitude modulations in the signal, thus degrading CR. Ripple is another cause. In addition, radars have tended toward multimode operation, where the pulse parameter are changed continuously, based on the operational scenario encountered. This is to optimize the radar signal for the type of target in each beam position. Also, the overall power radiated must be minimized to reduce prime power consumption and intercept of radar signals.
These modes require different duty cycles and pulse repetition frequencies, which can cause transmitter power supply fluctuations much larger than the expected power line ripple. DC power supply feedback loops can be too slow to respond to mode changes.
In the past, MTI stability was measured by two primary methods. The first is a delay line pulse cancellation procedure that subtracts the detected versions of the present RF pulse and a delayed version of a previous RF pulse. The method requires a lossy delay line that may distort the delayed pulse's amplitude and phase properties. Other problems with this method are that the exact delay time is hard to control and the range of usable delays is limited by loss considerations. This measurement procedure indicates only how the amplitude difference of the two pulses varies with time.
The second method for testing MTI stability measures both amplitude and phase variations by having the user adjust the phase relationship between the RF and local oscillator (LO) ports of a double balanced mixer. If the RF and LO are in phase, the mixer acts as an AM detector. A 90 degree phase difference between the ports causes the mixer to be a PM detector. Sometimes two sets of mixers and phase shifters are used to permit measurement of AM and PM variations simultaneously. There are at least two problems with this method. First, precision phase shifters are narrow band devices as far as their tuning range and bandwidth are concerned, and second, it is difficult to adjust the phase shifters to the exact 0 or 90 degree points.